A variety of barriers and other devices for attenuation of acoustic energy generated by a source have been heretofore suggested and/or utilized for a number of differing applications where complete enclosure of the source is either undesirable or impractical. Many, such as berms and fencing, do little more than provide absorbing structure adjacent to the source. In the case of barriers having a top edge, such as fencing and other panel-type designs, diffraction of acoustic or electromagnetic energy at the barrier edge can actually exacerbate the problem which the barrier is intended, at least in part, to resolve by decreasing the angle at which energy is launched.
It is well known that a variety of atmospheric conditions and/or effects can trap, scatter, reflect or refract energy, such as acoustic energy, and return that energy to the earth's surface, often some distance from the source. This is particularly the case with energy launched at relatively low angles (for example, less than about 45 degrees), since the steeper the angle of launch (relative to the horizon) the less it is likely such atmospheric conditions and/or effects will act to redirect the energy back toward the earth. It would thus be desirable to reduce such energy launched at low angles.
Heretofore known barrier designs have included various baffle-type designs (see U.S. Pat. No. 4,095,669), absorbing and or reflective designs (see U.S. Pat. No. 4,219,101), and complex geometries directed to shifting the phase of and/or refracting an acoustic wave (see U.S. Pat. No. 4,436,179). Such designs have not addressed the problem of diffraction of acoustic energy to lower launch angles at the upper edges of such barriers, and thus have not proven entirely satisfactory for many applications.
Other devices have been heretofore suggested and or utilized for silencing noise inherent in certain mechanical operations, for example exhaust system noise (see U.S. Pat. Nos. 2,652,127 and 4,872,528). Such devices have not, however, utilized the geometry suggested herein to resolve the problems addressed thereby.
At least one noise barrier has been suggested which utilizes controlled diffraction to deepen the acoustical shadow adjacent to the barrier by providing vertical, spaced, members adjacent to a source of noise designed to introduce destructive interference (see U.S. Pat. Nos. 4,175,639 and 4,228,867). This barrier design, however, actually seems to increase the intensity of sound launched at low angles (relative to barriers without the extensions), and thus does little to improve noise reduction at greater distances from the source.
Further improvement in such barriers and other devices could thus still be utilized.